The tuning of a microwave generator, e.g. an oscillator formed by a coaxial magnetron, is generally accomplished with the aid of a linearly displaceable element such as a piston, designed to modify the effective volume of a resonant cavity, having two terminal positions in which the operating frequency respectively lies at the lower and upper limits of its range. The change in frequency, however, is not proportional to the displacement of the tuning element. Thus, the instantaneous operating frequency cannot be directly ascertained from the magnitude of an output signal emitted by a conventional position sensor such as a resolver determining the angle of rotation of a crankshaft from a certain reference position. Since a crankshaft linked with the tuning element via a connecting rod or pitman is a convenient means for the adjustment of the generator frequency, it is desirable to obtain a precise correlation between that frequency and the crankshaft position.
Such correlation is important, for example, in a radar or other installation where a microwave receiver must be tuned, under the control of an output signal from a position sensor, to a frequency corresponding to that of an associated microwave transmitter whose operating frequency is varied in the aforedescribed manner.
Conventional systems for establishing a linear relationship between the frequency of a microwave oscillator, controlled by a linearly displaceable tuning element, and the output signal of a resolver, monitoring the angular position of a crank or cam operatively coupled with that element, make use of one or more pairs of eccentric gears in mesh with each other. Such devices are cumbersome and, upon prolonged use, tend to become inaccurate owing to wear of their gear teeth.